Final keyword and enumeration type

Catalog

final What is it?

final How to use it?

final matters needing attention

What are enumeration types

Format of enumeration type

Examples of enumeration types

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final What is it?

final The surface meaning of is unchangeable 、 Constant quantity 、 The final 、 Immutable meaning , It refers to the amount that cannot be changed .

stay java in final The modifier is a constant , And the variable name should be capitalized .

for example ：

public static final double PI = 3.14;

final How to use it?

① Modifying variables , By final Decorated variables must be initialized , After the initial value is assigned, it cannot be reassigned .

final int x1= 10000;

② Modification methods , By final The decorated method cannot override .

Created an animal class , There are ways to eat and sleep , The method of sleeping is final modification .

public class Animal {
    public void eat(){
        System.out.println(" Animals eat ");
    }
    public final void sleep(){
        System.out.println(" Animals sleep ");
    }
}

  Then create a dog class to inherit animal class .

public class Dog extends Animal{

}

Then override the method of the parent class in the dog class

  You can see , Dogs can only treat people who are not final Modified method rewriting , But you can call the parent class that is final The method of decoration .

public class Dog extends Animal{
    @Override
    public void eat() {
        System.out.println(" Dogs eat ");
    }
}

public class AnimalTest {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.sleep();
        dog.eat();
    }
}

 ③ decorator , By final Modified class , uninheritable .

④ Modify local variables , Variable can only be assigned once .

⑤ Decorate instance member variables , It can only be assigned once and must be assigned .

final matters needing attention

① By final The name of the modified constant , Generally, there are writing standards , All letters are capitalized .

② The variables in the interface are public static final Of .

③ Be careful not to final And finally、finalize() Wait to get confused .

What are enumeration types

stay Java in , By enum The type of keyword decoration is enumeration type , It's a special type of data , The reason why it is special is that it is not only a kind of type, but also has some special constraints more than a kind of type , Therefore, the existence of these constraints also creates the simplicity of enumeration types 、 Security and convenience . Enumeration is generally used in the following cases ： Define constants 、 Add new method 、 And switch Use a combination of 、 Implementation interface 、 Use interfaces to organize enumerations .

Format of enumeration type

public enum Enum name {

        Enumerate instances , These instances write capital names directly .

}

for example ：

public enum Sex{
  BOY,GIRL;
}

Examples of enumeration types

Enumeration type and switch Comprehensive application of ：

Create an enumeration type week class ：

public enum Week {
    MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY,SUNDAY
}

Create a test class , There is a way to show the day of the week .

public class WeekTest {
    public static void main(String[] args) {
        show(Week.MONDAY);
    }
    public static void show(Week week){
        switch (week){
            case MONDAY:
                System.out.println(" Monday ");
                break;
            case TUESDAY:
                System.out.println(" Tuesday ");
                break;
            case WEDNESDAY:
                System.out.println(" Wednesday ");
                break;
            case THURSDAY:
                System.out.println(" Thursday ");
                break;
            case FRIDAY:
                System.out.println(" Friday ");
                break;
            case SATURDAY:
                System.out.println(" Saturday ");
                break;
            case SUNDAY:
                System.out.println(" Sunday ");
                break;
        }
    }
}